The VP3 genes of the simian SA11, bovine NCDV, porcine OSU, and porcine Gottfried strains of rotavirus were studied by sequence analysis. The overall amino acid homology of the VP3 of two SA11 strains was only 82.7%; however the VP3 protein of one of these SA11 strains exhibited an homology of 98.4% with NCDV, suggesting that this SA11 VP3 was derived by inadvertent gene reassortment in another laboratory during contamination with a bovine rotavirus. The VP3 amino acid amino acid homology of OSU and other animal rotaviruses is approximately 80%. Surprisingly, the VP3 protein of Gottfried virus is closely related to the VP3 of human asymptomatic rotavirus strains in various ways including (1) nucleotide sequence and (2) neutralization by hyperimmune antisera or VP3 cross-reactive neutralizing monoclonal antibodies. A comparison of the deduced amino acid sequence of the VP3 of two virulent NCDV strains and an attenuated NCDV vaccine strain (RIT 4237), revealed only five amino acid substitutions which were scattered throughout the protein but did not involve the trypsin cleavage sites. One of the virulent strains and the attenuated strain differed in only one amino acid (residue 76) suggesting that a gene or a series of genes other than the VP3 sustained mutations which were responsible for attenuation of the candidate RIT vaccine strain of NCDV.